1. Chapter 15 – Gene Expression
Control of Gene Expression in Prokaryotes and Eukaryotes

2. Sample Pathway for Control of Tryptophan (trp) Expression
Promotor (RNA polymerase binding site)
transcription
translation
Enzyme 1
Gene 1
Each enzyme catalyzes the next series of reactions necessary for tryptophan production
Enzyme 2
Gene 2
Enzyme 3
Gene 3
Enzyme 4
Gene 4
Enzyme 5
Gene 5
2 Negative Feedback
shut-off systems
trp
mRNA (transcription unit)
DNA

3. Basic Vocabulary mRNA Transcription Unit:
- grouping of mRNA genes with related function
- 2 advantages ~
1. organization
2. easy regulation – one “on/off” switch can be used to control an entire cluster of related genes

4. Basic Vocabulary Operon:
- an entire gene-regulating system in PROKARYOTES, which includes the operator, promotor, and all corresponding genes

5. Basic Vocabulary Operator: - “on-off” switch to genes
- controls the access of RNA polymerase to genes
- found within the promotor region or between the promoter and gene-encoding regions
Let’s take a closer look at the tryptophan (trp) operon…an example of a “repressor” system

6. Overview of Repressor Trp Operon Components
Trp operon controls the production of the amino acid tryptophan
Overview of Repressor Trp Operon Components
r. Trp Repressor Gene (always “on”)
promotor
5 Genes needed for tryptophan production
rp. RNA Pol. binding site
o. Operator r.
rp. o.
Gene 1
Gene 1
Gene 3
Gene 4
Gene 5
RNA polymerase
mRNA transcription unit
mRNA for trp repressor
Enz 1
Enz 2
Enz 3
Enz 4
Enz 5
trp
Trp repressor protein translated in its “inactive” form

7. How is the trp operon turned off once enough trp is made?
Trp repressor gene
RNA pol binding site
operator
trp
trp
trp
trp
Inactive trp repressor protein
trp
Increased concentration of trp (co-repressor) increases chances of trp binding to allosteric site of inactive trp repressor

8. How is the trp operon turned off once enough trp is made?
RNA polymerse, therefore is physically blocked from transcribing genes for trp
Active trp repressor can now bind to operator

9. Why is this a repressor system?
The trp operon is a repressor operon, meaning gene expression of the operon is repressed by the presence of the co-repressor, tryptophan.

10. Watch this animation to fully understand and review this operon, then take the on-line quiz together as a class

11. Overview of Inducible Lac Operon
The Lac operon controls the production of the ß-galactosidase, an enzyme that catalyzes the hydrolysis (break-down) of lactose into glucose and galactose.
This is an inducible operon, meaning gene expression ß-galactosidase is stimulated by the presence of an co-inducer, lactose.

12. Inducible Lac operon promotor
Genes that promote ß-galactosidase production
crp: helps RNA pol. to bind
rp: RNA pol.binding site
o. operator
I. Lac Inducer I.
crp.
rp. o.
mRNA for inducer protein
RNA pol. blocked from ß-galactosidase transcription without lactose co-inducer
inducer protein in “active” form

13. TWO regulatory mechanisms used to turn on lac operon
Presence of lactose as a co-inducer
** reason?
ß-galactosidase is not needed unless lactose needs to be broken down
2) Low amounts of glucose
Recall that lactose breaks down into glucose and galactose. Low glucose levels signals the cell for more lactose to be broken down.
Both conditions must be met for the lac operon to turn on.

14. TWO regulatory mechanisms used to turn on lac operon
RNA polymerase can bind only with the help of CRP transcription factor
RNA pol.
AlloLactose
cAMP
CRP Transcription Factor
1) Allolactose co-inducer attaches to allosteric site of inducer protein, inactivating it
2. cAMP attaches to CRP transcription factor when glucose is scarce

15. Lac Operon Animation
Watch this animation to fully understand and review the lac operon!

16. How are genes controlled in eukaryotes?
Regulation of Chromatin Structure
Pre and post Transcriptional Regulation
Pre and Post Translational Regulation

17. Regulation of Chromatin Structure
N-terminus (amino group) of histone proteins face outwards from nucleosome
Tails are thus able to be modified chemically

18. Regulation of Chromatin Structure
Methylation  Promotes condensation
Phosphorylation  can prevent condensation, if phosphorylation is adjacent to methyl group
Histone Acetylation – neutralizes (+) charges on tails, which prevents binding to adjacent nucleosome  loose chromatin structure results, allowing for increased transcription

19. Pre-Transcriptional Regulation
Similar to methods used in bacterial operons, using proteins that inhibit or promote binding of RNA pol.
Distal and Proximal Control Elements
Proteins involved include:
Transcription factors
Activators
Mediator Proteins

21. Post-Transcriptional Regulation
RNA Processing
– differential/alternative splicing can produce different 20 mRNA transcript
Differential splicing redefines which RNA segments are considered introns and which are exons

22. Post-Transcriptional Regulation
Time of mRNA degradation can vary

23. Pre-Translational Regulation
Initiation of Translation
- can be blocked by regulatory proteins that prevent ribosome binding
- shortened polyA tails in mRNA prevents translation (polyA tails can be added during appropriate time)
- global regulatory control of all mRNAs in cell

24. Post-Translational Regulation
During protein processing, folding
Timing of protein degredation can vary
Proteasomes degrade proteins that are tagged by ubiquitine molecules